The present embodiments relate to modifying the local intensity of x-ray radiation.
In the case of examinations with the aid of x-ray beams, the organs of a patient generally have very different properties with respect to the absorption of the applied x-ray radiation in the region to be examined. Thus, for example, in the case of chest imaging, the attenuation in the region in front of the lungs is very high due to the organs arranged there, while it is very low in the lungs themselves. In order to obtain a meaningful image and to minimize patient radiation exposure, the applied radiation dose is often set depending on the region in such a way that no more x-ray radiation than necessary is supplied. A larger x-ray radiation dose may be applied in the regions with high attenuation than in regions with less attenuation. Additionally, there are applications in which only part of the region to be examined needs to be imaged with high diagnostic quality. The surrounding parts are important for orientation but not for the actual diagnosis. These surrounding regions can therefore be imaged with a lower radiation dose in order to reduce the overall applied radiation dose.
In diagnostic x-ray instruments, collimators and attenuators are positioned between the x-ray source and the patient in order to minimize the beam exposure of the patient. Often, the settings for collimators and attenuators are selected once, usually manually, by the operator of the x-ray equipment prior to x-ray imaging. The settings are often only possible in discrete steps and cannot be varied during the imaging.
The x-ray beam shape and the x-ray beam profile are typically set in three steps. The measurement beam is initially hardened during pre-filtering as a result of the soft or low-energy components of the x-ray beam being absorbed by a filter, as the low-energy components do not contribute to imaging. The thickness of the filter is often set once in discrete steps prior to the imaging via the insertion of copper disks with different thicknesses. U.S. Pat. No. 4,688,424 discloses a filter arrangement with holes. By moving the arrangement along the beam axis, the absorption of the x-ray beam is set and the intensity of the x-ray beam is varied.
If strongly absorbing tissue or materials situated near weakly absorbing tissue lie in an image field, the beam intensity within the image field is adapted by wedge-shaped filters in a second step. To this end, only a few standard geometries are available and optimum adaptation to the patient is possible to a limited extent. As an alternative, U.S. Pat. No. 5,881,127 discloses a device in which the beam profile is shaped by metallic cylinders. This device requires a multiplicity of mechanical components, making the integration of the device, for example into a C-arm, much more difficult. Moreover, it is very complicated to design the structure in such a way that the mechanical components do not leave artifacts in the space.
In a third step, a collimator restricts the image field to the region relevant for the diagnosis. The restriction is typically only possible in the form of rectangles of various sizes or other standard geometries. EP Patent No. 2395918 describes an adjustable aperture, through which the corners of a rectangle are rounded off to different extents in successive image recordings.